The background of the invention can be described most clearly, and hence the invention can be taught most effectively, by subdividing this section in three subsections. The first subsection will provide some general background regarding the role of crosslinked (and especially stiff and strong thermoset) particles in the field of the invention. The second subsection will describe the prior art that has been taught in the patent literature. The third subsection will provide additional relevant background information selected from the vast scientific literature on polymer materials science and chemistry, to further facilitate the teaching of the invention.